Ba0.5Sr0.5Co0.8Fe0.2O0-delta (BSCF) is a material with excellent oxygen ionic and electronic transport properties reported by many research groups. In its cubic phase, this mixed ionic-electronic conducting (MIEC) perovskite is a promising candidate for oxygen permeation membranes. For this application, its long-term stability under operating conditions (especially temperature and oxygen partial pressure) is of crucial importance. The present work is focused on the thermal stability of the BSCF cubic phase in the targeted temperature range for applications (700 ... 900 degrees C) in light of previous studies in literature reporting a reversible transition to a hexagonal phase somewhere below 900 degrees C. To this end, single phase cubic BSCF powders were annealed at different temperatures over varying periods of time. Phase composition was subsequently analysed by X-ray diffractometry (XRD) in order to determine both the temperature limit and the time-scale for the formation of the hexagonal phase. Additionally, the long-term behaviour of the electrical conductivity was examined on bulk samples at 700 degrees C, 800 degrees C and 900 degrees C over several hundreds of hours, showing a prolonged decrease at 800 degrees C. The decrease in electrical conductivity at this temperature was also examined on bulk samples with different grain sizes, showing a more pronounced decrease the smaller the average grain size. Coexistence of both phases (cubic and hexagonal) could also be shown for 700 degrees C, however with a different phase equilibrium than at 800 degrees C. (C) 2011 Elsevier B.V. All rights reserved.